A color image forming apparatus of an electrophotographic method is well known. In this method, one photosensitive member undergoes developments of respective colors using a plurality of developers, color images are superposed and formed on a single transfer material by repeating exposure, development, and transfer processes a plurality of number of times, and these color images are fixed to obtain a full-color image.
However, this method must repeat three (four if black is used) image forming processes to obtain a print image on one sheet, resulting in a long image forming time.
As a system that can cope with this drawback, a so-called tandem system which superposes visual images obtained for respective colors using a plurality of photosensitive members to obtain a full-color print via a single paper feed operation is known. According to this tandem system, the throughput can be greatly improved. On the other hand, a color discrepancy program has occurred due to misalignments of respective colors on a transfer material resulting from errors of the positional precisions and diameters of photosensitive members, and the positional precision errors of optical systems, and it is difficult to obtain a high-quality full-color image.
Various measures against this color discrepancy have been proposed. For example, Japanese Patent Application Laid-Open No. 64-40956 (parent reference 1) discloses a technique which forms a test toner image on a transfer material or a transfer belt which forms a transfer unit, detects the formed image, and corrects the optical path of each optical system or corrects the image write start position of each color based on the detection result.
Japanese Patent Application Laid-Open No. 8-85237 (patent reference 2) discloses the following technique. The output coordinates of image data of respective colors are converted into those free from any registration errors. After that, based on the converted image data of respective colors, the positions of modulated light beams are corrected by an amount less than the minimum dot unit of a color signal.
However, the method disclosed in patent reference 1 poses, e.g., the following problems.
First, in order to correct the optical path of the optical system, a correction optical system including a light source and f-θ lens, a mirror in the optical path, and the like must be mechanically moved to adjust the position of the test toner image. For this purpose, high-precision movable members are required, resulting in high cost. Furthermore, since it takes much time until correction is completed, it is impossible to frequently perform correction. However, an optical path length difference may change along with an elapse of time due to temperature rise of mechanical components. In such case, it becomes difficult to prevent color discrepancy by correcting the optical path of the optical system. Second, in order to correct the image write start position, it is possible to conduct misalignment correction of the left end and upper left portion but it is impossible to correct the tilt of the optical system and to correct any magnification errors due to the optical path length difference.
The method disclosed in patent reference 2 poses, e.g., a problem of a large calculation volume since color discrepancy correction amounts must be calculated for all pixels. FIGS. 1A and 1B show an example. An image shown in FIG. 1A has a constant density value. In order to obtain an image shown in FIG. 1B by applying arbitrary color discrepancy correction to this input image, the density values corresponding to all pixels must be calculated. For this reason, the calculation volume becomes large, and the arrangement of a processing system becomes complicated.